Updat README.md to add vi/emacs keybindings as a feature and fix a typo in example code
Created by: jwbowen
Problem: vi and emacs keybindings are now supported, but missing from the feature list.
Solution: I added the fact that set -o (vi|emacs)
is now supported by ion.
State: Ready
Blocking/related: Issue #289 (closed) fixed by commit ce0a196 and issue #288 (closed)